Transcript Document
Enhanced Da H-mode on Alcator C-Mod
presented by J A Snipes
with major contributions from
M Greenwald, A E Hubbard, D Mossessian,
and the Alcator C-Mod Group
MIT Plasma Science and Fusion Center
Cambridge, MA 02139 USA
Seminar IPP Garching
Garching, Germany
7 May 2002
Global Features of EDA H-Mode
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EDA H-modes have:
Good energy confinement
H89 ~ 2
Low particle confinement
no impurity accumulation
Low radiated power
No large ELMs
Steady State (>8tE)
Obtained with Ohmic or ICRF
heating, 1 < PRF< 5 MW
Highly attractive reactor
regime (no ELM erosion)
Similar to LPCH-mode (JET)
and type II ELM regimes
A. Hubbard
Temperature and Density Profiles in
EDA H-mode
• Steep edge temperature and density gradients
• Moderately peaked temperature profile
• Flat density profile
Quasi-Coherent Signature of EDA H-mode
Enhanced Da emission in EDA H-mode
f ~100 kHz Quasi-Coherent density and
magnetic fluctuations always found in
EDA H-mode in the steep gradient edge
QC mode well correlated with reduced
particle and impurity confinement
No large Type I ELMs found on C-Mod
Only small irregular ELMs sometimes
found on top of the enhanced Da emission
M. Greenwald
Edge Pedestal and Fluctuation
Diagnostics
A. Hubbard
Quasi-Coherent Mode seen in Density
Fluctuations in EDA H-modes
• Quasi-coherent edge mode
always associated with EDA
H-Mode
• After brief ELM-free period
(~20 msec), mode appears
• Frequency in lab frame
decreases after onset (
~100 kHz in steady state)
– change in poloidal rotation
• Reflectometer localizes mode
to density pedestal
Y. Lin
Phase Contrast Imaging measures
kR ~ 6 cm-1 (l~1 cm)
• Frequency range 60-250 kHz
• Width DF/F ~ 0.05-0.2
A. Mazurenko
•PCI measures k radially at top
and bottom of plasma.
k R ~ 2 k for typical equilibria
k s
0.1
Steady Edge Pedestals in EDA
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EDA pedestal characterized by
steep pressure gradients
Pedestal parameters obtained
from tanh fit to measured
Thomson scattering profiles
Moderate pedestal Te (< 500 eV)
and high collisionality n* > 2
Steady-state conditions
throughout ICRF pulse
Quasicoherent mode observed by
reflectometer channel that views
plasma region near the middle of
the pedestal
D. Mossessian
Conditions Favoring EDA
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EDA formation favored by:
– Moderate safety factor
• q95 > 3.5 in D
• q95 > 2.5 (or lower) in H
– Stronger shaping
• d > 0.35
– Higher L-mode target density
• ne > 1.21020 m-3
– Clean wall conditions
(boronization)
Seen in both Ohmic and ICRF
heated discharges
Seen with both favorable and
unfavorable drift direction.
M. Greenwald
Higher density at L-H favours EDA
Low density, ELM-free
Higher density, EDA
Da
Da
ne
ne
•Actual threshold may be in neutral density, local ne or gradient or
collisionality (all are correlated; n*ped < 1 at low ne, 5-10 at high ne)
• 1.21020 m-3 quite low for C-mod. ~0.15 nGW , low ne limit ~0.9 1020
A. Mazurenko
EDA/ELM-free Operational Boundaries
EDA favors high q95 > 3.5 1
and moderate edge
150 < Teped < 500 eV
ELM-free plasmas are more
likely at low q95 and at lower
densities and hence higher
edge temperatures
0.6 MA < Ip < 1.3 MA
4.5 T < Bt < 6 T
1 MW < PRF < 5 MW
D. Mossessian
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M. Greenwald, Phys. Plasmas 6, 1943 (1999)
EDA/ELM-free Operational Boundaries
EDA favors high q95 > 3.5 1
and high edge collisionality
n*ped > 2
ELMy H modes occupy the
same q-n* region as EDA
ELM-free plasmas are more
likely at low q95 and at lower
collisionality
Collisionality n*ped
calculated on 95% yn (top of
the pedestal)
D. Mossessian
1
M. Greenwald, Phys. Plasmas 6, 1943 (1999)
Edge Gradients Challenge MHD Limit
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Edge electron profiles from high
resolution Thomson scattering
– assume Ti = Te
Modeling shows gradients are
~30% above the first stability
ballooning limit with only ohmic
current.
– Edge bootstrap current
increases stability limit
No Type I ELMs
(PRF5 MW, P12 MPa/m)
– Small ELMs when bN1.2
D. Mossessian
EDA Pedestal Pressure Increases with Ip
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Thomson pedestal electron
pressure gradient in EDA
increases strongly with plasma
current
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Dashed curves are
0.4
pe 2.8I 1.7
P
p sol
J. Hughes
Time evolution of Te, ne pedestals studied
using power ramps
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RF input power continuously
variable, ramped slowly up and
down.
Te, ne measured with ms time
resolution by ECE,
bremsstrahlung array.
Strong hysteresis in net P.
H-mode threshold in Tedge is
found.
Te pedestal varies in height and
width with P
ne pedestal independent of P
(above LH threshold).
A. Hubbard
Small ELMs appear at high input power
Small, bipolar ELMs in Da
at ~ 600 Hz
Plasma exhaust visible on
divertor probe saturation current
ELMs observed in fast magnetic
coil signal
D. Mossessian
QCM exists at moderate Pped and Teped
ELMy
EDA
When Teped 400 eV broadband low
frequency fluctuations observed in
the pedestal region
QC mode reappears when edge is
cooled
ELMs replace the QC mode at high
pedestal Te
D. Mossessian
EDA/ELM-free Boundary in Pped vs Teped
QCM is not observed when
Te >450 eV
ELMy regime exists in high Te,
high Pped region
D. Mossessian
Probe Measurements Confirm Mode Drives
Particle Transport
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Langmuir probes see mode
when inserted into pedestal
(only possible in low power,
ohmic, H-modes)
Amplitude up to ~50% in n, E
Multiple probes on single head
yield poloidal k~4-6 cm-1, in
agreement with PCI
– Propagation in electron
diamagnetic direction
Analysis of n E shows that
the mode drives significant
radial particle transport across
the barrier, G~ 1022 /m2 s
Plumes from probe gas puffs
show Er < 0 at mode location.
(Er > 0 at larger radii).
ne
1 mm
G n E
B. LaBombard
Particle Diffusion Increases with
Quasi-Coherent Mode Amplitude
• Particle source calculated
with Lyman-a emission, ne(r),
and Te(r)
• Effective particle diffusion:
DEFF = (Source - dN/dt)/ n
• As QC mode strength increases:
– Deff increases
– X-ray pedestal width (~Dimp)
increases.
M. Greenwald
QCM has a strong magnetic component
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Pickup coil added to fast-scanning
Langmuir probe.
Frequency of magnetic component
is identical to density fluctuations.
B ~ 3 104 T implies mode
current density in the pedestal ~10
A/cm2 (~10% of edge j).
Mode is only observed
within ~ 2 cm of the LCFS
Mode is NOT seen on the wall and
limiter coils that are 5 cm outside
the LCFS (at least 1000x lower)
J. Snipes
Magnetic QCM amplitude decreases
rapidly with radius
• Scanning magnetic probe
nearly reaches the LCFS
• Mode decays as
~
~
B BLCFS exp( kr ( r rLCFS ))
• Local QCM kr~1.5 cm-1
10 cm above the outboard
midplane
• Differs from Type III ELM
precursor kr~0.5 cm-1 seen
on the limiter probes
J. Snipes
QCM Poloidal Mode Structure
Frequency sweeps from > 200 kHz
to ~ 100 kHz just after L-H transition
Strong magnetic component only
observed within ~2 cm of LCFS
kr k 1.5 cm-1 (l 4 cm) near
the outboard midplane
Assuming a field aligned
perturbation with k B 0 , k is
expected to vary with position as
k1 / k 2 ( R2 / R1 )2 ( B 2 / B1 )
consistent with PCI kR ~ 6 cm-1 along
its vertical line of sight near the core
J. Snipes
QCM Toroidal Mode Structure
QCM is sometimes observed on a
toroidal array of outboard limiter
coils
When the outer gap 1 cm
Toroidal mode number
15 < n < 18
At q95 = 5, for a mode resonant at
the edge this implies
75 < m < 90
which is consistent with
<k> ~ 4 cm-1
J. Snipes
Toroidal mode number
Comparison with other
‘small ELM’ regimes
EDA H-mode shares some characteristics of other steady regimes
without large ELMS.
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Low Particle Confinement regime on JET
– Appears similar to EDA, but not easily reproduced.
Quasi-coherent Fluctuations on PDX
– Fluctuations similar to those in EDA, present in short bursts in most Hmodes. Coexisted with ELMs.
Type II or Grassy ELMs on DIII-D, JT60U, Asdex UG
– Conditions in q, d very similar to EDA
– Similar to small ELMs seen in EDA at high bN?
– Does a quasi-coherent mode play a role in these regimes?
Quiescent H-Mode on DIII-D
– Globally similar, but longer wavelength mode, different access
conditions (esp density/neutrals).
A. Hubbard
LPCH-mode on JET Similar to EDA
EDA H-mode in C-Mod
LPCH-mode in JET
J. Snipes
Bout Simulations of the QCM
X.Q. Xu, W.M. Nevins, LLNL
BOUT simulations find an X-point resistive ballooning mode that
is driven in the edge steep gradient region
has a similar magnetic perturbation amplitude and radial structure as the QCM
has a similar dominant k ~ 1.2 cm-1 at the outboard midplane as the QCM
Physical origin of EDA, fluctuations
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Since pedestal profiles are not much different in EDA, ELM-free H-modes,
it seems likely to be the mode stability criteria which change with q,d, n*
etc.
One possibility is that EDA is related to drift ballooning turbulence.
Diamagnetic stabilization threshold scales as m1/2/q.
A lower q threshold was found for EDA in H than D.
Initial scalings of QC mode characteristics show
n s k 0.1 0.2
s
n Dn
Electromagnetic edge turbulence simulations by Rogers et al have shown a
feature similar to QC mode, with k 2 / D p .
Gyrokinetic simulations of growth rates (GS2 code) are in progress.
M. Greenwald
Summary
• EDA H-mode combines good energy confinement and
moderate particle confinement in steady state, without large
ELMs
• Edge pedestals have few mm widths, gradients above first
stable limit; but stable with bootstrap currents
• Quasicoherent pedestal fluctuations QCM in density, potential
and B are a key feature of EDA and only occur when:
n*ped > 2, Pped < 1.2x106 Pa/(Wb/rad), Teped <450 eV
• At higher Pped, high Teped QC mode is replaced by small
grassy ELMs
• The observed fluctuations drive significant particle flux
• QCM’s are tentatively identified as resistive ballooning modes